1. Field of the Invention
The present invention relates to an apparatus and a method for identifying a wafer.
2. Description of the Background Art
When production control and data control of a semiconductor device are exercised by wafer or lot, it is necessary to identify a wafer that is the foundation of a semiconductor device. In general, for identification of a wafer, lot No. and wafer No. as identification mark are engraved on the surface of the wafer by e.g., a laser.
In the background art, first, the lot No. and the wafer No. engraved on the surface of the wafer are read as graphic information by direct observation of the engraved portion with a CCD camera and secondly a process for recognizing the graphic information as character information (referred to as "OCR process" hereinafter) is performed, to identify the wafer.
FIG. 9 is a partial cross section showing a wafer which is engraved with the lot No. and wafer No. as identification mark on its surface by using a laser. As shown in this figure, engraved portions 7 are formed at the depth of e.g., about 0.2 .mu.m from the surface of a wafer 1. When the lot No. or the like is engraved at the depth of about 0.2 .mu.m or more from the surface of the wafer 1, the engraved mark can be read direct with the CCD camera.
FIG. 10 is a partial cross section showing the wafer 1 and an oxide film 3 formed on the surface of the wafer 1 of FIG. 9. In many cases, the oxide film 3 or the like is formed on the wafer 1 in a process for manufacturing a semiconductor device, where a concave portion 13 is produced in a region of the surface of the oxide film 3 above the engraved portion 7 due to the level difference of the engraved portion 7 and no concave portion is produced in other region.
FIG. 11 is a cross section showing an exemplary wafer identification apparatus in the background art. A light irradiation unit 8 irradiates the surface of the wafer 1 provided with the engraved portion 7 with a light from diagonally above the engraved portion 7. On the other hand, a CCD camera 10 is disposed so that its light-receptive axis should be out of the direction of a regularly-reflected light La, and receives only a reflected light Lb that is reflected on the engraved portion 7. Setting the light-receptive axis of the CCD camera 10 out of the direction of the regularly-reflected light La provides more intense contrast between the engraved portion 7 and the peripheral portion, to offer higher reading performance than simple read of the content of the engraved portion 7 with the CCD camera 10 without light irradiation.
Even if the engraved portion 7 is formed at the depth of about 0.2 .mu.m from the surface of the wafer 1, however, the concave portion 13 is shallower when the oxide film 3 is formed on the surface of the wafer 1 as shown in FIG. 10, and sufficient contrast can not be obtained between the concave portion 13 and its peripheral portion by using the wafer identification apparatus of FIG. 11, as well as by direct read of the surface configuration of the concave portion 13 direct with the CCD camera 10. As a result, the wafer can not be identified.
Considering that the concave portion 13 is shallower than the engraved portion 7 when the oxide film 3 is formed on the surface of the wafer 1, it may be proposed that the engraved portion 7 should be formed deeper from the surface of the wafer 1. But it is impossible to form the engraved portion 7 unlimitedly deeper because more dust would be generated in forming the engraved portion 7, causing deposition of the dust on the surface of the wafer 1 or chemical agents used in the process for manufacturing the semiconductor device would be collected in the engraved portion 7.